Evaporative coolers (aka swamp coolers, desert coolers, wet air coolers, and swamp boxes) cool air by evaporating water, unlike AC systems that use absorption or compression. Water takes in significant heat before evaporating, so that heat can be absorbed from the surrounding air as the water turns to vapor. This process is especially valuable in dry climates, since it increases nearby air humidity.
The systems date back to an ancient Egyptian and Persian technology called windcatching, which used shafts in roofs to channel incoming outside air down to run over underground water, releasing the resulting cool air into buildings. Today, the descendants of these passive systems cool people with powered versions in arid places from Iran to Arizona.
Early refrigeration also used evaporative cooling principles, by storing food goods in jars surrounded by water and covered with a cloth. The human body performs a version of evaporative cooling, as sweat turns into vapor on the skin. And compression refrigeration systems use the same principles, just in a closed system that doesn't release any steam, instead cycling it back into water and blowing away the heat released in that process with a fan that takes the hot air outdoors.
Evaporative coolers are much cheaper than refrigerated air conditioning, cost less to install and maintain, and rely only on water instead of dangerous chemicals. They also keep air fresh and filtered, and even reduce static electricity by raising humidity.
On the other hand, they're not as effective as refrigerated systems, don't work as well in environments that are already humid, and use a large amount of water. Moreover, the humidity they release can accelerate corrosion, shortening the lifespan of electronic devices, and it reduces the effectiveness of human sweat as a coolant.
Poorly maintained evaporative coolers can be a breeding ground for mosquitos, bacteria, and mold. The name swamp cooler is attributed to the pungent scent from algae in some early modern models.